Sheet stretcher



D. M. GILMAN SHEET STRETCHER June 14, 1966 Filed May 6, 1964 INVENTOR.DAVID M GIL/MAN BY W $344M ATTO R N EYS thereof; and

United States Patent 3,255,540 SHEET STRETCHER David M. Gilman,Brookline, Mass, assignor to Graphic Equipment of Boston, Inc., Boston,Mass., 2 corporation of Massachusetts Filed May 6, 1964, Ser. No.365,313 6 Claims. (Cl. 38-102.91)

The present invention relates to sheet stretching and more particularlyto an improved means for rapidly and efliciently stretching orflattening material in sheet form.

Stretching of materials is particularly important in the silk screenart. Commonly silk sheets or screens are stretched on a frame,nonprinting areas are blocked off, and ink pressed through the portionsof the screens corresponding to printing areas while they are held inthe stretched position. In prior art procedures for stretching silkscreens, various clamps or key pieces are used which often requirewooden constructions. Such constructions are diflicult to assemble anddisassemble thus increasing the time required to change screens andconsequently the cost of such methods.

Accordingly, it is an important object of this invention to provide asheet stretcher which rapidly and eficiently stretches a sheet andmaintains it in a stretched condition for any desired period of time.

It is another object of this invention to provide a sheet stretcher inaccordance with the preceding object which has relatively few parts andcan be operated by hand with case.

It is still another object of this invention to provide a sheetstretcher in accordance with the preceding objects which can beconstructed entirely of metal and consequently has a long trouble-freeworking life.

According to the invention the sheet stretcher has an enclosing framehaving an upper sheet bearing edge providing opposing edge portions.Opposing clamps are provided on or adjacent opposing edge portions ofthe frame and are mounted for movement toward and away from the edgeportions so that a sheet can be mounted in the clamps and stretched overthe bearing edge when the clamps are moved away from the edge.Preferably each of the clamps are mounted on the frame by upwardlyextending threaded spindles. The clamps each preferably comprise a lowerclamp member defining an elongated channel opening on an upper surfaceand extending inwardly into the lower clamp member toward the frame. Anupper spring clamp member has an overturned lip which is designed to fitwithin the longitudinally extending channel and clamp the sheet betweenwalls of the channel.

Other features, objects, and advantages of the invention will be betterunderstood and appreciated from the following detailed description ofone embodiment thereof selected for purposes of illustration and shownin the accompanying drawings, in which:

FIG. 1'is a top plan view of a preferred embodiment of this invention;

FIG. 2 is a perspective view of a portion of one side FIG. 3 is a topplan view of a sheet to be stretched by the device of FIG. 1.

As best shown in FIG. 1, the sheet stretcher 14) of this invention has agenerally polygonal stretching frame 11 with four clamping means 12mounted on the sides of the frame. The frame 11 is preferablyconstructed of metal angles joined at their corners by conventionalbrazing,

welding or bolting techniques. Upstanding walls or flanges 13, 14, 15and 16 provide an upper rectangular or square bearing edge 21 extendingaround the frame and preferably lying a plane. Depending or outwardlyflared base flanges 17, 18, 19 and 20 extend outwardly of walls 13, 14,15 and 16, respectively.

In the preferred embodiment of the invention four identical stretcherclamps 12 are each mounted insliding relationship with the upstandingwalls of the frame by threaded spindles 23 as best shown in FIG. 2.Preferably, each of the clamps is individually moveable toward ledge 29.The lower clamp members are preferably aluminum extrusions althoughother metals can be used. An elongated channel or recess 30 is providedin the lower clamp member 24 along its length with an end 31 spacedcloser to the side wall of the frame than the opening 32 on the surface25. T he recess 30 is defined by side walls 33 and 34 and an outerrabbeted recess 35 adjacent the opening 32.

An upper clamp or spring member 36 is provided for each lower stretcherclamp member 24. The upper clamp member 36 is preferably formed of ametallic strip and has an elongated overturned lip 37 having an outerconfiguration designed to fit within the recess 30. A substantially flatstrip portion 38 of the upper clamp member 36 is adapted to lie onsurface 25 of the lower clamp member 24. The locking and clamping actionofeach stretcher clamp 12 will be further described with reference tothe operation of the sheet stretcher.

The threaded spindle 23 is threadably engaged with a verticallyextending threaded bore in stretcher clamp 12. Preferably threeidentical threaded spindles are used for each stretcher clamp 12although one, two or more threaded spindles can be used. The structureand operation of only one spindle will be described since all of thespindles are identical. The lower end of the threaded spindle 23 passesthrough the base flange 17 as best shown in FIG. 2 and has a threadedlocking collar 54 and an end knob 56 on the lower side of flange 17.Preferably the portion of the spindle between collar 54 and knob 56 isunthreaded and adapted to rotate freely in a hole in the flange 17 ofslightly greater diameter than the diameter of the spindle. Thus,rotation of the knob 54 rotates the spindle and consequently causesmovement of stretcher clamp 12 along the axis of the spindle 23 towardor away from the plane of the upper bearing edge 21.

The particular threaded spindle arrangement may vary considerably. Forexample, in some cases the spindle may be fixed to the clamping memberand a collar arrange ment may be provided for moving the spindle alongits axis with relation to the lower flange 17. During movement of thestretcher clamp 12 the projection 27 preferably engages the side wall 13tending to stabilize the clamp 3 12 and leg 28 acts as a positive stopor ultimate limit for stretching of a sheet in the stretcher frame.

A sheet of silk screen to be stretched in the sheet stretcher of thisinvention is shown at 40 in FIG. 3. Preferably the silk screen 40 isgenerally rectangular in shape having cut-away corner portions withpoints 41, 42, 43 and 44 corresponding to the corners of the frame 11.Flaps 45, 46, 47 and 48 are adapted to overly the upper bearing edge 21and be clamped to the stretcher clamps 12. Although it is preferred tostretch sheets having the configuration shown in FIG. 3 in a rectangularframe, square, rectangular or irregular shaped sheets may also bestretched over'the bearing edge 21.

Turning now to the operation of the sheet stretcher of this invention,stretcher clamping members 12 are raised to their uppermost positionpreferably at or slightly below the plane defined by the bearing edge21. Upper clamp members 36 are removed and a silk screen 40 placed overthe frame 11 with portions 41, 42, 43 and 44 thereof overlying thecorners of the frame. The flap portions 45, 46, 47 and 48 overlie theclamping members with ends of the silk screen 40 extending to orslightly beyond the outermost edges of the stretcher clamps. Upper clampmembers 36 are then placed in position with each strip 38 lyingsubstantially parallel to each upper surface and each lip 37 enteringrecess and having silk screen separating it from the recess and lying inthe position shown in dotted lines at 49. In this mounting position, thethreaded spindles 23 have their upper ends substantially at the lowerend of threaded bores 52. Preferably after all of the clamping uppermembers 36 are positioned as described, knobs 56 of each stretcher clampmember are rotated and the stretcher clamps 12 move downwardly over thethreaded spindles 23. As the upper ends of the threaded spindles 23gradually approach the upper surfaces 25, the sheet 40 is stretched overthe bearing edge 21 with each of the flaps being stretched at portions50 and directing an upward force against inner edge 26 of upper clampmembers 36 pivoting them into preselected positions with respect to thelower clamping members as best shown in FIG. 2. The upward force exertedon the inner edges 26 of the upper clamping members 36 cause portions 38thereof to act as levers with the inner ends of lips 37 pressingportions of the silk screen against walls 33 of the recess and otherportions 39 of the lips 37 acting as fulcrums and pressing secondportions of the screen against the locking groove walls 35 as seen inFIG. 2. This action effectively locks the screen to the stretcher clamp.Each stretcher clamp 12 can be individually movedtoward the base flangesstretching the silk screen 40 the desired amount and holding it forindefinite periods of time.

In some cases collar 54 can comprise a threaded collar which may 'bemoved upwardly on the spindle 23 to lock the stretcher clamp member 24in positive position when the correct degree of stretch is attained.However, even without the use of a threaded collar the tension in thesilk screen tends to positively stabilize the position of the clampingmembers.

When a screen is to be changed, the threaded spindles 23 are rotatedmoving the stretcher clamp members 12 to their uppermost positionreducing the upward force on the edge 26 of upper clamping members 36and allowing ease of removal of both the upper clamping members and thesilk screen. It is a feature of this invention that each of the clampingmembers is individually adjustable assuring ease of adjustment tostretch along various lines of stretch in the sheet.

Although specific embodiments of this invention have been shown anddescribed, many variations thereof are possible. For example, althoughthe recess 30 extends inwardly at an acute angle to upper surface 25 andthe planeof the sheet 40 at an acute angle, this angle may be varied atleast up to 90. The particular means for moving the stretcher clampstoward and away from the planes of the bearing edge 21 may varyconsiderably, as may the cross section of the clamping members and metalangle braces. Although it is preferred to use metal such as aluminum forall of the elements of the inventive structure, other material may besubstituted in certain cases. The specific sheet material beingstretched may be silk, cotton, wool, textiles, paper and others.

Consequently, the breadth of this invention is to be construed aslimited only by the spirit and scope of the appended claims.

What is claimed is:

1. A sheet stretcher comprising a generally polygonal frame,

said frame having a sheet bearing edge lying substantially in a planeand defining opposing edge portions, first and second opposing stretcherclamping means for clamping opposing ends of a sheet to be stretched,said first and second stretcher clamping means each lying on opposingedge portions and means mounting said first clamping means for movementtoward and away from said plane whereby a sheet may be mounted on saidfirst and second clamping means and stretched over said bearing edgewhen said first clamping means is moved away from said plane, said firstand second stretcher clamping means each comprising a lower clampingmember and a pivotable upper clamping member,

said upper clamping member having a lip dimensioned to be received in arecess defined by said lower clamping member and maintained thereinwithout the need for additional fastening means when pivoted intopreselected position with respect to saidlower clamping member by aclamped sheet during said movement away from said plane.

2. A sheet stretcher in accordance with claim 1 wherein said polygonalframe comprises an angle member defining an upstanding wall and anoutwardly flared bottom flange with an upper portion of said upstandingwall defining said sheet bearing edge.

3. A sheet stretcher in accordance with claim 1 wherein said lowerclamping member is mounted for said movement by a threaded spindlehaving one end passing into said lower member and a second end mountedon said frame whereby rotation of said spindle causes said movement,

said lower clamping member having a surface defining and elongatedrecess extending into said clamping member at an angle to said plane.

4. In combination with a sheet stretcher comprising a substantiallypolygonal frame having an upstanding substantially polygonal walldefining an upper sheet bearing edge lying substantially in a plane,

the improvement comprising a plurality of opposing clamping meansmounted on said frame for movement toward and away from said plane,

said clamping means comprising an elongated lower clamping member havingan upper surface defining a longitudinally extending recess therein,

said recess opening on said surface and extending inwardly into saidmember toward said wall to a recess end closer to said wall than saidrecess opening,

and an upper clamping member comprising a strip having a longitudinallyextending over-turned lip dimensioned to fit within said recess with alayer of sheet material separating said lower member from said uppermember whereby when a force is applied to said strip upon movement ofsaid clamping means away from said plane, said lip clamps said sheet insaid recess.

5.'The improvement of claim 4 wherein said clamping means are mounted onsaid frame by a plurality of threaded spindles threadably engaged withsaid clamping means so that rotation of said spindles in a firstdirection causes movement away from said plane and rotation of saidspindles in a second direction causes movement of said clamping meanstowards said plane.

6. The improvement of claim 4 wherein said clamping means are composedof a metal and said polygonal frame is square.

References Cited by the Examiner UNITED STATES PATENTS 6 FOREIGN PATENTS4/1953 Belgium. 6/1933 Switzerland.

FRANK J. COHEN, Primary Examiner.

1. A SHEET STRETCHER COMPRISING A GENERALLY POLYGONAL FRAME, SAID FRAMEHAVING A SHEET BEARING EDGE LYING SUBSTANTIALLY IN A PLANE AND DEFININGOPPOSING EDGE PORTIONS, FIRST AND SECOND OPPOSING STRETCHER CLAMPINGMEANS FOR CLAMPING OPPOSING ENDS OF A SHEET TO BE STRETCHED, SAID FIRSTAND SECOND STRETCHER CLAMPING MEANS EACH LYING ON OPPOSING EDGE PORTIONSAND MEANS MOUNTING SAID FIRST CLAMPING MEANS FOR MOVEMENT TOWARD ANDAWAY FROM SAID PLANE WHEREBY A SHEET MAY BE MOUNTED ON SAID FIRST ANDSECOND CLAMPING MEANS AND STRETCHED OVER SAID BEARING EDGE WHEN SAIDFIRST CLAMPING MEANS IS MOVED AWAY FROM SAID PLANE, SAID FIRST ANDSECOND STRETCHER CLAMPING MEANS EACH COMPRISING A LOWER CLAMPING MEMBERAND A PIVOTABLE UPPER CLAMPING MEMBER, SAID UPPER CLAMPING MEMBER HAVINGA LIP DIMENSIONED TO BE RECEIVED IN A RECESS DEFINED BY SAID LOWER